(1) Technical Field
The present invention relates to a system and method of fabricating organic thin-film transistors (OTFTs). More specifically, the present invention relates to the employment of a flexible, conductive particle-polymer composite material for ohmic contacts (i.e. drain and source).
(2) Description of Related Art
Organic electronics have recently attracted tremendous interest within the microelectronics industry for potential use in a wide range of commercial applications, such as large area flexible displays as well as other consumer electronics applications including smart cards, radio frequency identification (RFID) tags and electronic paper. This technology utilizes molecular and polymeric semiconducting compounds in place of traditional materials such as silicon. Thin-film transistors (TFTs) and organic light-emitting diodes (OLEDs) can be fabricated from different organic and polymeric compounds using a variety of low-temperature, low-cost processing techniques including solution casting, spin coating and ink jet printing.
Steady improvements in materials and processes over the last few years have brought the performance of organic-TFT (OTFT) devices in line with that of amorphous silicon devices. Unlike amorphous silicon, however, the organic semiconducting layers can be deposited at room temperature, allowing the organic semiconducting layers to be printed directly onto heat-sensitive structures. These TFTs can be used in applications where low-cost or mechanical flexibility is required.
Beyond the aforementioned commercial applications, development of robust organic electronics for extreme environmental conditions is also of interest for space and military applications. Possible applications to human exploration of space include flexible displays, flexible health sensing electronics, and chemical/biological sensing electronics which could be incorporated into protective clothing. Potential applications for robotic missions include distributed sensors for large area surface diagnostics and metrology on inflatable/deployable structures. Examples of distributed sensors include temperature sensors, strain sensors, shape sensors, tear detection sensors or even sensors to control piezoelectric actuators. In the case of sensors distributed on large-area surfaces of inflatable/deployable structures, integration of rigid silicon-based chips within such a structure is impractical due to problems with assembly, die thinning, mechanical stresses, thermal mismatch and die attach. For these applications, very high-density electronics are not required (due to the very large areas available) and could benefit from the integration of flexible (OTFT) based sensor and sensor electronics.
Previous work in the area of organic transistors has focused on the use of either 1) vacuum-deposited noble metal contacts or 2) solution-deposited intrinsically conducting molecular or polymeric contacts to the form the drain and source contacts.
Use of vacuum-deposited noble metal contacts, such as gold or platinum, obviates one of the main benefits of organic electronics, i.e., low-cost processing based on solution or printing techniques. Vacuum deposition requires the use of techniques, such as sputtering or evaporation, instead of less expensive solution-based processes such as spin coating, casting or printing. Second, the use of gold or platinum for coating large area devices is potentially expensive, both from a stand point of materials and processing equipment. Again, this approach runs counter to the perceived low-cost benefit of organic electronics. Furthermore, the adhesion of gold to many organic materials is very poor.
Some recent work has been carried out regarding intrinsically conducting molecular-based or polymeric-based contacts such as polyaniline and TTF-TCNQ, examples of such work include C. J. Druru, C. M. J. Mutsaers, C. M. Hart, et al., “Low-cost All-polymer Integrated Circuits, Applied Physics Letters, vol. 73, pp. 108-110, July 1998 and K. Saito and S. Kobayashi, “Deposition of Organic Electrodes Based on Wet Process for Organic Devices,” Applied Physics Letters, vol. 80, pp. 1489-1491, February 2002. Unfortunately, these materials tend to exhibit high resistivities and poor overall performance, are prone to reaction with the surrounding environment, and are potentially unstable over time.
Within the field of organic electronics, the search continues for molecular and polymeric semiconducting materials, which are soluble in common solvents, stable under ambient conditions and exhibit high carrier mobilities. In addition, within the field of organic electronics, studies are being conducted regarding materials that can be used as the gate dielectric and as the drain and source contact layers for organic-thin-film transistor (OTFT) devices. The drain and source contacts are particularly critical, as they form a direct ohmic contact with the organic semiconductor. Furthermore, it is desirable that these contacts be stable over time with respect to the semiconductor and dielectric interfaces as well as the ambient environment, particularly when biased at high voltages (up to a 100 V differential between drain/source and source/gate). Finally, for many emerging applications, it is desirable that these contacts be flexible, and may be able to be processed at low temperatures while maintaining adequate adhesion to the underlying substrate.
To achieve an ohmic contact to the organic semiconductor, the work function of the contact should be well matched to that of the semiconductor. Previous work has focused on the use of noble metal contacts and conducting polymers to form these contacts. However, this type of processing can be expensive and can result in poor quality devices. What are needed are contacts which are inexpensive, flexible, stable, form ohmic contacts to common organic semiconductors, and can be readily processed without the need for expensive processing equipment, such as vacuum deposition equipment.